Fixed-coordinate mode: validate remotes whose upstream the suite cannot write

Add a mode that validates a remote repository whose upstream the suite cannot write to, by reading a pre-existing coordinate instead of seeding a run-scoped one.

Deferred by S08 §Out of Scope. Related: #47 (closed).

Problem

S08's fixture seeding model pushes each fixture to --upstream-url and reads it back through --registry-url. That requires an upstream the run credential can write. Against the most common real-world remote, one proxying npmjs.org, Maven Central, or Docker Hub, no such write is possible, so the run must pass --upstream-free-only and exercises only the Absence and Write refusal classes. Neither says anything about proxying, which is the feature under test.

S08 §Which remotes this suite validates states the boundary. This issue is the work that moves it.

Why now rather than "when a consumer needs it"

S08 deferred this with "revisit when a consumer needs it". Review of !231 (merged) pointed out that the trigger is already met by the spec's own premise: a public-upstream remote is described there as the more common deployment, so the consumer is not hypothetical. Filing it makes the deferral tracked rather than a footnote.

Scope

  • A flag surface for naming a pre-existing coordinate per format, and its validation rules.
  • Which of S08's assertion classes the mode can carry, and how a run reports the ones it cannot.
  • The catalog consequence: whether these are new rows, or existing remote rows running under a second fixture source.

What it does not buy

Not the whole gap, and a plan should not assume otherwise. S08's cold-start property is what the Repeat read class rests on: a row that asserts the second read differs from the first needs the first to be a known miss, and a pre-existing coordinate cannot guarantee that because any earlier reader may have warmed the cache. So maven.remote.repeat-read, npm.remote.repeat-read, oci.remote.repeat-read, and maven.remote.cold-fill-header-subset stay unvalidatable under a fixed coordinate. What the mode buys is the Relay class.

Acceptance

  • S08 amended: the §Out of Scope entry becomes a defined mode, with the cold-start limitation kept as a stated constraint of the mode rather than a reason to defer it.
  • The flag surface follows docs/dev/configuration.md's checklist and lands in S04's inventory the way --upstream-url did.
  • The catalog decision recorded before any row is implemented.